I've been in the process of writing a vba program and I needed to be able to programmatically draw the arcs to approximate the ellipse the same way that autoCAD does. I searched and searched and couldn't find anything that says how autoCAD comes up with the approximation that it does. I found this post in my search.
After a lot of messing with the problem I finally figured out how autoCAD does it. So if anyone is still interested in this I've attached a drawing file (it's a 2007 version) that shows the math and geometry behind autoCAD's pellipse =1 ellipse approximation.
Hope this helps someone besides me.
Let me know if you have any questions.
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